doc: Build the docs without Buildstream installed
Description
I'm working on a RPM package for Fedora.
Currently the docs can't build in an RPM as they require Buildstream to be installed, but it is obviously not installed yet.
More generally, it seems desirable to be able to build the docs before installing Buildstream, whether from a tarball or from a Git clone.
This merge request adds the right folders to Python's syspath, so that the docs building process can find Buildstream and its plugins.
The CI is changed as well to not install Buildstream any more before building the docs, to ensure this does not regress.